They fly apart, because they are improperly installed.

Normally the backplate is cracked because the user has installed a prop that pushes the blades back against the backplate, causing it to bend and stress. This will just as often happen with aluminum spinners as well, particularly those with "lips" along the edges.

A plastic spinner tends to be lighter, and has lower inertial forces acting upon it to tear it apart, so it may actually fare better than some of the aluminum ones.

I've seen both types come apart, and it has always been because of improper installation/usage.